Transplantation of tissue into the mammalian central nervous system has become a widely used technique for exploring brain plasticity and developmental (Freed et al., 1985). Recent studies suggest that brain grafting may also have clinical utility (Madrazo et al., 1987). In the rat, the animal where most grafting research has taken place, grafts can be implanted using simple stereotaxically controlled injections (Perlow et al., 1979). This approach is very effective for implantation of tissues in the ventricles (Freed et al., 1981) or for implantation of dissociated cells (Bjorklund et al., 1980). Other procedures are available for placing grafts into the ventricular wall (Morihisa et al., 1984; Madrazo et: al., 1987). When larger tissue fragments are implanted into the brain parenchyma, however, it becomes necessary to force the tissue into place by injecting substantial volumes under sufficient pressure to displace host brain tissue. This procedure may, therefore, alter the graft implantation site or even damage the grafted tissues.
Implantation of solid tissue fragments into brain parenchyma has often been relatively ineffective (Freed et al., 1986) for several reasons. For example, squirting or pushing tissue through a long needle may disrupt or damage the tissue. On the other hand, placing tissue into the brain with a spring or other holding device, which has been done with parkinsonian patients, presents potential problems associated with leaving a foreign object in the brain and may cause excessive disruption of host tissue (Backlund et al., 1985a,b). Direct visual placement, another approach to human work, is possible in only a limited number of brain sites (Morihisa et al., 1984; Madrazo et al., 1987); and in larger brains, such as those of monkeys and humans, these problems become more pronounced.